Bishop didn't contest the allegations that he communicated with the girlfriend, his defence attorney Birney Bervar said after a short hearing in Hawaii. But he said the letters were love letters, showing the case was about love and not espionage.

"Nothing substantive or substantial in there. They were just love letters wishing her a happy birthday and love and good life," Bervar said.

Bervar said Bishop had asked another resident of the halfway house where he has been living to send the letters and set up an email account so he could contact his girlfriend.

Bishop, who is about 60, had been arrested in March at US Pacific Command headquarters, where he was working as a contractor.

Prosecutors charged Bishop, who was a lieutenant colonel in the US Army Reserve, with one count of communicating national defence information to a person not entitled to receive it, and one count of unlawfully retaining national defence documents and plans.

Authorities say Bishop gave his girlfriend, a graduate student in her late 20s, secrets about US nuclear weapons, missile defences, war plans, early warning radar systems and other issues.